A new study warns that transgender persons taking hormones may face an increased risk of blood clots and strokes.

Researchers from Kaiser Permanente indicate that cross-sex estrogen, used by transfeminine persons, appear to increase the risk of venous thromboembolism (VTE) and ischemic stroke. The findings were published July 10 in the Annals of Internal Medicine.

The research involves a cohort study looking at transgender members of integrated health care systems who declared transgender status from 2006 through 2014. Researchers then matched ten men and 10 female cisgender (born biologically female) subjects to the transgender patients using a number of factors, such as age and ethnicity.

The study involved 2,842 transfeminine members (those transitioning to female), and 2,118 transmasculine members (those transitioning to male), as well as 48,686 cisgender men and 48,775 cisgender women, with a mean follow-up of up to four years. The researchers looked for incidence of venous thromboembolism, heart attack and stroke through 2016.

According to the findings, transfeminine subjects had a higher risk of venous thromboembolism, which can increase the risk of pulmonary embolism and deep vein thrombosis, than cisgender men and cisgender women. In addition, those transfeminine participants who began hormone therapy during the follow-up period showed more pronounced venous thromboembolism and ischemic stroke risks.

The researchers indicated that the data was insufficient to make conclusions regarding transmasculine subjects.

“The patterns of increases in VTE and ischemic stroke rates among transfeminine persons are not consistent with those observed in cisgender women,” the researchers determined. “These results may indicate the need for long-term vigilance in identifying vascular side effects of cross-sex estrogen.”